1. Field of the Invention
The present invention relates to a single-phase induction motor, and more particularly to a single-phase induction motor provided with means for preventing a small amount of current from flowing to a start device composed of a PTC or the like when the motor runs in normal mode, so as to prevent a reduction in the efficiency of the motor due to power consumption by the start device in normal running mode of the motor.
2. Description of the Related Art
First, a conventional single-phase induction motor is described with reference to FIG. 1.
FIG. 1 is a circuit diagram of a conventional single-phase induction motor. As shown in FIG. 1, the conventional single-phase induction motor is driven by a power source E, and includes a main winding M, an auxiliary winding S, a run capacitor Cr connected in series with the auxiliary winding S, and a Positive Thermal Coefficient element (hereinafter, referred to as a “PTC”) connected in parallel with the run capacitor Cr. A start capacitor Cs may be connected in series with the start device.
The PTC, which is used as a start device, is an element whose resistance varies depending on temperature. The PTC has a high resistance at high temperature and a low resistance at low temperature.
The conventional single-phase induction motor configured as described above operates in the following manner.
When the motor starts up with power supplied from the power source E, the resistance of the PTC is low, allowing a current provided to the auxiliary winding S to pass through the PTC and the start capacitor Cs. Accordingly, a high start torque is produced when the motor starts.
On the other hand, when the motor runs in normal mode after a predetermined time from the start of the motor, the temperature of the PTC is increased so that it has a very high resistance, thereby opening a start device connection line through which the PTC is connected to the circuit. As the start device connection line is opened, the current provided to the auxiliary winding S flows through the run capacitor Cr, so that the main winding M, the auxiliary winding S, and the run capacitor Cr produce magnetic fields, and the interaction between the produced magnetic fields and a rotor (not shown) causes the rotor to rotate at synchronous speed.
Ideally, the resistance of the PTC reaches infinite when the motor is in normal running mode, so that no current flows through the PTC. However, practically, a small amount of leakage current flows through the PTC when the motor is in normal running mode, and the leakage current causes unnecessary power consumption by the PTC, thereby reducing the overall efficiency of the motor.
One solution to this problem is a single-phase induction motor including means for preventing power consumption by a PTC when the motor is in normal running mode.
FIG. 2 is a circuit diagram of a conventional single-phase induction motor provided with means for preventing power consumption by a start device in normal running mode. As shown in FIG. 2, the single-phase induction motor includes a main winding M, an auxiliary winding S, a search coil 1, and a triac 2. The main winding M produces time-varying magnetic flux, and the produced magnetic flux induces a voltage in the search coil 1. The voltage induced in the search coil 1 is applied to the gate of the triac 2, which is a semiconductor switching element.
When the motor starts up, a large amount of current is provided to the main winding M, so that a high voltage is induced in the search coil 1, thereby turning on the triac 2. As the triac 2 is turned on, a current provided to the auxiliary winding S flows through the triac 2 and a resistor R, which corresponds to a start device.
On the other hand, when the motor runs in normal mode, a small amount of current is provided to the main winding M, so that a low voltage is induced in the search coil 1, thereby turning off the triac 2. As the triac 2 is turned off, all the current provided to the auxiliary winding S flows through the run capacitor Cr, thereby preventing power consumption by the PTC (corresponding to the resistor R) when the motor runs in normal mode.
However, in the case of using the above means for preventing power consumption by the PTC when the motor is in normal running mode, it is difficult to install the search coil and also to install an additional device for inducing a voltage through time-varying magnetic fields, consequently reducing the installation and overall efficiency of the motor.